Why Do Waterfalls Appear White?
The color of water varies with the ambient conditions in which that water is present. While relatively small quantities of water appear to be colorless, pure water has a slight blue color that becomes deeper as the thickness of the observed sample increases. The hue of water is an intrinsic property. It is due to selective absorption and scattering of blue light. Dissolved elements or suspended impurities may give water a different color.
Absorptions in the visible spectrum are due to excitations of electronic energy states in matter. Water is a simple three-atom molecule, H2O, and all its electronic absorptions occur in the ultraviolet region of the electromagnetic spectrum.
The water molecule has three fundamental modes of vibration. Two stretching vibrations of the O–H bonds in the gaseous state of water occur at v1 = 3650/ cm and v3 = 3755/ cm. Absorption due to these vibrations occurs in the infrared region of the spectrum. The absorption in the visible spectrum is due mainly to the harmonic v1 + 3v3 = 14,318/ cm, which is equal in value to a wavelength of 698 nm. In a liquid state at 20°C, these vibrations are red-shifted by hydrogen bonding, resulting in red absorption at 740 nm, and other harmonics such as v1 + v2 + 3v3 giving red absorption at 660 nm. [6] The shape of the absorption curve for heavy water (D2O) is similar to that of light water. It is shifted further towards the infrared end of the spectrum because the vibrational transitions have a lower energy. For this reason, heavy water does not absorb red light, and thus, large bodies of D2O would lack the characteristic cyan color of the more commonly found light water (1H2O). [7]
Water
flowing in a canal or a slow-moving river is colorless and laminar. It is a
uniform and smooth flow. When we watch this, the reflection of light is from
one angle.
The
significant change that occurs is that of the change in flow pattern. It
changes from laminar to turbulent flow.
- Fluid particles follow smooth paths in layers.
- Each layer moves smoothly past the adjacent layers with little or no mixing. [1]
- There are no cross-currents perpendicular to the direction of flow. [2]
- The motion of the particles of the fluid is in order, with particles close to a solid surface moving in straight lines parallel to that surface. [3]
- High momentum diffusion and low momentum convection are laminar flow characteristics.
- When it falls from a higher place, the flow pattern changes from laminar to turbulent.
- Turbulent flow is rapid and non-uniform, which changes flow parameters.
- The water gets aerated.
- All layers of water get mixed in a turbulent flow.
- The dissolved oxygen in the water evaporates and releases oxygen to form millions of water bubbles, which are responsible for the white color of the water.
The
crashing noise of a waterfall is the sound of air trapped in water, escaping
while falling.
Turbulent
water can also result in the formation of foam in slower-moving parts of the
river.
I told
you there are millions of water bubbles, and light undergoes multiple
refraction and reflection from the surface of these bubbles randomly, in different
angles and directions. The overall effect is the scattering of light. When
light shines on it, all the bubbles look white. The scattered light appears
white because of diffuse reflection.[4]
Lakes and oceans appear cyan for several reasons. One is that the water surface reflects the color of the sky, which ranges from cyan to light azure. Depending on the sky's brightness, the water body appears more a shade of azure than a cyan color. [8][9] Water in swimming pools with white-painted sides and bottoms will appear cyan. In the case of indoor pools, there is no reflection of the sky, but it appears cyan. The deeper the pool, the more intense the cyan color becomes. [10]
A part of the light received by the Ocean surface reflects, and most of the remaining light penetrates the water surface, interacting with water molecules and other substances. Water molecules can vibrate in three different modes when they interact with light. The red, orange, and yellow wavelengths of light are absorbed, so the remaining light seen is composed of green, cyan, and blue wavelengths.
It is the main reason the ocean's color is cyan. The relative contribution of the reflected skylight and the light scattered back from the depths depends strongly on the observation angle. [11]
The azure color of the water is due to the reflection of the sky, and the cyan color comes from the intrinsic color of water scattered back up to the surface by small suspended particles.
Scattering from suspended particles also plays a role in the color of lakes and oceans, causing the water to look greener or bluer in different areas. A few tens of meters of water will absorb all light. So, without scattering, all bodies of water would appear black. Because most lakes and oceans contain suspended living matter and mineral particles, light from above is scattered, and a part of it reflects upwards. Scattering from suspended particles would give a white color, as with snow, but because the light first passes through many meters of cyan-colored liquid, the scattered light appears cyan. In extremely pure water—as is found in mountain lakes, where scattering from particles is very low—the scattering from water molecules also contributes a cyan color. [12][13]
Color of glaciers
Relatively small amounts of regular ice appear white because plenty of air bubbles are present.
In glaciers, pressure in the accumulated snow causes the trapping of air bubbles. Large quantities of water appear cyan. Therefore, a large piece of compressed ice, or a glacier, would also appear cyan. [4]
Color of water samples
The color of water is due to the dissolved and particulate material. For instance, dissolved organic compounds called tannins can result in dark brown colors, or algae floating in the water can impart a green color. [14] Color variations can be measured using a standard color scale.
The color of a water sample:
- Apparent color is the color of a body of water reflected from the surface and consists of color from both dissolved and suspended components. Change in apparent color may be due to the variations in sky color or the reflection of nearby vegetation.
- The collected water sample needs to be purified (either by centrifuging or filtration) to get the right color of the water. Pure water tends to look cyan. We can compare it with pure water samples with a predetermined color standard or the results of a spectrophotometer. [15]
Testing for color can be quick and easy and often reflects the amount of organic material in the water. Inorganic components like iron or manganese can also impart color.
Watercolor can reveal physical, chemical, and bacteriological conditions. In drinking water, green can indicate copper leaching from copper plumbing and can also represent algae growth. Blue can also indicate copper or might be the siphoning of industrial cleaners in the tank of commodes, commonly known as backflowing. Reds can be signs of rust from iron pipes or airborne bacteria from lakes, etc. Black water can indicate the growth of sulfur-reducing bacteria inside a hot water tank set to too low a temperature. This water usually has a strong sulfur or rotten egg (H2S) odor and is easily corrected by draining the water heater and increasing the temperature to 49 °C (120 °F) or higher. [4] The impurity indication color spectrum can make identifying and solving cosmetic, bacteriological, and chemical problems easier.
Water quality and color
The color of the water does not necessarily indicate that the water is not drinkable. Water with high clarity is generally more cyan due to the low concentration of particles or dissolved substances. Filtration reduces color-causing particulate substances. Color-causing dissolved substances such as tannins are only toxic to animals in large concentrations. [16]
Other factors can affect the color:
- Particles and solutes can absorb light, as in tea or coffee. Green algae in rivers and streams often lend a blue-green color. [4]
- Particles in water can scatter light. The Colorado River is often muddy red because of suspended reddish silt in the water—this gives the river its name, from Spanish Colorado, color, red. Some mountain lakes and streams with finely ground rock, such as glacial flour, are turquoise.
The Ancient Indian Wisdom of Veda considers the life-giving contributions of water a part of the divine, water as God, Varuna, and the color of Varuna is blue. [4] As per Vastu rules, we, the Kerala people, consider water a source of positive energy and a synthesis balance of all energies. To learn more about Kerala architecture, please read my article, https://hubpages.com/education/slow-aging.
In the Gayatri associated with Varuna, the word "Neela purusha" comes in the second line, which calls the water deity the blue one. [4]
Diffuse sky radiation due to Rayleigh scattering in the atmosphere along one's line of sight gives distant objects a cyan or light azure tint.
Do
you know what diffuse reflection is?
Diffuse
reflection is the reflection of light at many angles from a surface. It scatters light in different directions when it encounters a
rough or irregular surface.
References:
1. Streeter, V.L. (1951-1966) Fluid Mechanics, Section 3.3 (4th edition). McGraw-Hill
2. ^ Jump up to a b Geankoplis, Christie John (2003). Transport Processes and Separation Process Principles. Prentice Hall Professional Technical Reference. ISBN 978-0-13-101367-4. Archived from the original on 2015-05-01.
3. ^ Noakes, Cath; Sleigh, Andrew (January 2009). "Real Fluids". An Introduction to Fluid Mechanics. University of Leeds. Archived from the original on 21 October 2010. Retrieved 23 November 2010.
4 Wikipedia
5 Pope; Fry (1996). "Absorption spectrum (380–700nm) of pure water. II. Integrating cavity measurements". Applied Optics. 36 (33): 8710–8723. Bibcode:1997ApOpt..36.8710P. doi:10.1364/ao.36.008710. PMID 18264420. S2CID 110616256 ^ Braun, Charles L.; Smirnov, Sergei N. (1993), "Why is water blue?" (PDF), Journal of Chemical Education, 70 (8): 612–614, Bibcode:1993JChEd..70..612B, doi:10.1021/ed070p612
7^ WebExhibits. "Colours from Vibration". Causes of Colour. WebExhibits. Archived from the original on 23 February 2017. Retrieved 21 October 2017.
Heavy water is colorless because all of its corresponding vibrational transitions are shifted to lower energy (higher wavelength) by the increase in isotope mass.
9 ^ "Common Misconceptions About Oceans — Polar Oceans — Beyond Penguins and Polar Bears". 18 July 2011. Retrieved 5 July 2022.
10 ^ Rossing, Thomas D.; Chiaverina, Christopher J. (1999). Light science: physics and the visual arts. Springer Science+Business Media. pp. 6–7. ISBN 978-0-387-98827-6.
11 ^ Braun & Smirnov 1993, p. 613: "... the relative contribution of the reflected skylight and the light scattered back from the depths is strongly dependent on observation angle."
12 ^ Pope, Robin M.; Fry, Edward S. (20 November 1997). "Absorption spectrum (380–700 nm) of pure water. II. Integrating cavity measurements". Applied Optics. 36 (33). The Optical Society: 8710–8723. Bibcode:1997ApOpt..36.8710P. doi:10.1364/ao.36.008710. ISSN 0003-6935. PMID 18264420. S2CID 11061625.
13^ Morel, Anclré; Prieur, Louis (1977). "Analysis of variations in ocean color1". Limnology and Oceanography. 22 (4). Wiley: 709–722. Bibcode:1977LimOc..22..709M. doi:10.4319/lo.1977.22.4.0709. ISSN 0024-3590
15 ^ Wetzel, R. G. (2001). Limnology (3rd ed.). New York: Academic Press.
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